Each relay (called also expansion) boosts the SMS sending rate of your main ProWebSms gateway App by:
• 60 SMS / hour on Android 4.1, 4.2, 4.3, 4.4, Android 5 and recent versions
• 100 SMS / hour on older versions of Android.
Without an expansion Relay, ProWebSms is limited to 30 SMS per 30 minutes (on Android 4.1 and greater) to avoid a confirmation popup from showing.